Corn plants (Zea mays L.) can be self-pollinating or cross pollinating. Self pollination for extended time periods produces homozygosity at almost all gene loci, forming a uniform population of true breeding progeny. These are inbreds. Crossing two homozygous inbreds produces heterozygous gene loci in hybrid plants and seeds.
Maize plant breeding is a process to develop improved maize germplasm in an inbred or hybrid. Hybrids are developed with inbreds which are developed by selecting corn lines and self pollinating these lines for several generations to develop homozygous pure inbred lines. Two inbred lines are crossed and hybrid seed is produced. One inbred is emasculated and the pollen from the other inbred pollinates the emasculated inbred. The emasculated inbred, often referred to as the female, produces the hybrid seed F1. Emasculation of the inbred can be done by detasseling the seed parent, or the inbred could have a male sterility factor which would eliminate the need to detassel the inbred.
Whether the seed producing plant is emasculated due to detasseling or CMS or transgenes, the seed produced by crossing two inbreds in this manner is hybrid seed. This hybrid seed is F1 hybrid seed. The grain produced by a plant grown from a F1 hybrid seed is referred to as F2 or grain. Although, all F1 seed and plants, produced by this hybrid seed production system using the same two inbreds should be substantially the same, all F2 grain produced from the F1 plant will be segregating maize material.
The hybrid seed production produces hybrid seed which is heterozygous. The heterozygosis results in hybrid plants, which are robust and vigorous plants. Inbreds on the other hand are mostly homozygous. This homozygosity renders the inbred lines less vigorous. Inbred seed can be difficult to produce since the inbreeding process in corn lines decreases the vigor. However, when two inbred lines are crossed, the hybrid plant evidences greatly increased vigor and seed yield compared to open pollinated, segregating maize plants. An important consequence of the homozygosity and the homogenity of the inbred maize lines is that all hybrid seed produced from any cross of two such elite lines will be the same hybrid seed and make the same hybrid plant. Thus the use of inbreds makes hybrid seed which can be reproduced readily.
The ultimate objective of the commercial maize seed companies is to produce high yielding, agronomically sound plants that perform well in certain regions or areas of the Corn Belt.